a) Field of the Invention
The invention is directed to a device for welding thermoplastic plastic parts forming a hollow body by means of laser radiation as is known generically from JP 11348132.
b) Description of the Related Art
Thermoplastic plastic parts (parts to be joined) can be connected to one another by different welding methods in which different kinds of energy, such as laser radiation, are used to fuse the parts to be joined.
DE 19510493 describes a thermoplastic housing and a method for welding thermoplastic housing parts by means of a laser. The basic idea consists in that the two housing parts to be welded, at least in the region of the joint zone (entire area in which a melt is formed), are made on the one hand from a material which is extensively transparent to the laser radiation and, on the other hand, from a material which extensively absorbs the laser radiation, and the laser radiation is directed to the housing in such a way that it initially penetrates the material that is transparent to the laser radiation and subsequently impinges on the material which absorbs the laser radiation.
The fact that the energy needed for producing the weld connection can reach the joint zone via one of the parts to be joined allows the possibility of a free arrangement of the weld seam in the housing, i.e., the weld seam need not extend in a plane, but may also extend three-dimensionally or in the interior of the housing. In principle, this allows a much more flexible possibility for shaping the housing parts and joint zone compared with other methods known from the prior art. Moreover, even housings with very thick walls can be welded. The parts to be joined can be butt-joined or lap-joined.
The quality of a weld formed in this way, which is intended in particular to ensure a tight connection between the joined parts, but also should be formed without melt flashing so as to avoiding finishing work, is influenced by various steps in the prior art. This may involve special arrangements of the housing parts in the area of the joint zone which influence the shape of the melt and/or producing an engagement (frictional or positive connection) between the housing parts to be joined during the welding process in order to achieve a defined joining gap.
It is stated in DE 195 10 493 that the quality of the weld can be enhanced by applying pressure to the joining zone until cooled. In so doing, the pressure should preferably be applied adjacent to the focus of the laser beam, so that the laser radiation can penetrate into the workpiece without hindrance. Also expressed is the idea of using transparent clamping means in the area of the laser beam in order to be able to carry out the application of pressure directly in the area in which the laser radiation acts. The pressure action should preferably follow the movement of the laser beam along the joint zone.
A disadvantage in generating applied force, as suggested herein, is that the parts to be joined must be shaped in such a way that clamping means can make contact in the area of the joint zone. Readjustment of the clamping means requires constant tightening and loosening which, in particular, requires additional expenditure on apparatus. Moreover, the time in which force is applied in each instance is very limited when readjusting the clamping means unless the duration over which the process is carried out is increased.
In DE 4432081, devices for fixing the parts to be joined for welding by radiation energy are criticized as disadvantageous because of their wastefulness. The stated object is to find a solution in which plastic parts can be welded together by means of radiation energy without devices for fixing the parts to be connected. The solution consists in that the parts to be joined are fixed with respect to one another by a positive engagement, particularly in the effective direction of force, starting with the formation of the melt (melt pressure), e.g., by means of a snap-in connection or a thread. It is also suggested that the parts to be joined be fixed relative to one another by an “inner frictional engagement such as screws or magnets”. Fixing must then be dimensioned only in such a way that the local welding pressure moving along the contour of the seam as the beam is guided is absorbed.
However, this means that special steps must be taken in every case with respect to the construction of the two parts to be joined. Even if this did not contradict the function and desired arrangement, it necessarily means increased preliminary fabrication costs and additional material costs, e.g., for the above-mentioned magnets or screw connections. In the case of large-series manufacture and mass production, the increased expenditure may far exceed the expenditure on apparatus for fixing, which can be reused over and over.
DE 4225679 is directed to a method and a device for welding the upper part and lower part of a housing, wherein a filter disk is introduced between the housing parts to be welded. In this case, particularly strict requirements are imposed on the dimensional tolerance of the joint gap and on the strength of the joint seam. Continuous quality control must be carried out. In the described device, the individual parts to be welded must be arranged one on top of the other and pressed together by a pressing device, while a laser beam is guided along the joint seam to be welded. No mention is made in the description regarding the pressing device or the contact pressing force generated by it.
JP 11348132 also discloses a method and a device for laser welding. In this case, also, two housing parts forming a hollow body are to be welded together by means of a laser. Specifically, the hollow body mentioned herein is a lamp with a reflector as lower part of the housing and a light disk as upper part of the housing. The reflector is arranged horizontally in a stationary manner so as to be supported by its edge on the cylinder wall of a holding device. A pressing device supported on the light disk presses the light disk firmly on the edge of the reflector. On the one hand, the holding device and the pressing device in cooperation perform the function of the pressing or clamping devices known in the art in that they press together the two parts to be joined at their joining surfaces forming the joint zone; on the other hand, it serves to hold and accordingly position the housing relative to the laser which is guided along the joint zone for carrying out the welding process. Compared with the solutions known in the art for pressing together the joint surfaces, this solution is characterized in that the housing is held and positioned by it in addition.
However, the disclosed device is narrowly limited in its application with respect to the shape of the housing and the position of the joint zone in the housing. Further, the shape of the pressing device must be adapted to the surface shape of the light disk.
It is not known from the prior art whether a device of this kind performs additional functions beyond those of pressing together the joint surfaces and holding the housing.